1. Field of Invention
The present invention relates to a mode-locked fiber laser technology. More particularly, the present invention relates to a mode-locking device in the polarization additive pulse mode-locked (P-APM) fiber laser, in which a mode locking unit is integrated into a solid, small volume structure and assembled into a compact mode-locked fiber laser, decreasing ambient interferences.
2. Description of Related Art
Being likely to be interfered by ambient interferences to lose mode-locking, P-APM fiber laser, published in 1992, was not commercialized immediately. Along with the development of highly-doped fiber laser, the length of fibers of fiber laser has been getting shorter, slightly decreasing the sensitivity of fiber laser to ambient interferences, which makes it be possible to commercialize this kind of mode-locked fiber laser. However, the production of a stable node-locked fiber laser relies on the design of whole laser system. It is a great task to design a laser to be compact so as to prepare a stable environment to resist ambient interferences.
FIG. 1 is a schematic drawing of the architecture of a conventional P-APM fiber laser. A pump laser 100 provides a laser source. A wavelength division multiplexer (WDM) 102 couples the pump laser into a gain fiber 104, which is doped with rare earth element, e.g. Er. The pump laser excites the rare earth element to produce laser radiation entering the mode locking unit through lenses 106 and 116. The mode locking unit consists of two groups of retardation waveplates 108 and 114 and a polarization dependent isolator 112. A group of retardation waveplates 118 consists of two λ/4 waveplates; the other group of retardation waveplates 114 consists of a λ/4 waveplate and a λ/2 waveplate. The laser radiation with a component in polarized state is coupled out through a polarizing splitter 110.
If components of the mode locking unit are disposed loosely in the laser system, not only the assembling size of the whole system is very big, but also the respective components are likely to be interfered by various ambient factors, degrading the performance of laser. The mode-locking mechanism of P-APM is likely to be interfered by ambient factors, for example, pressure, shake, and temperature variation. If the volume of the mode locker is sizeable, it is hard to design a compact laser system resisting the interferences of the above factors.